Process of concentrating aqueous



United States Patent 9 25.252 PROCES F CONCENTRATDIG AQUEOUS SIELHJASULS Raymond Renter, ()rland Park, and Alfred J. Tozydlo, Oak Lawn, ilL,nssignors to Nalco Chemical Company, a corporation ofDelawnre N0Drawing. Original No. 2,929,790, dated Mar. 22, 1960, Ser. No. 551,52Q,13cc. 7, 1955. Appiication for reissue July 6, E961, No. 124,304

4 Claims. (Si. 252 313) Matter enclosed in heavy brackets appears in theoriginal patent but forms no part of this reissue specification; matterprinted in italics indicates the additions made by reissue.

The present invention relates to a process for concentrating aqueoussilica sols. More particularly, it relates to a more simplified methodof concentrating aqueous silica sols than has been heretofore known.

Aqueous colloidal silica sols have been known for a number of years andhave been the subject of extensive scientific investigation. These solsare generally prepared by removing all but a small portion of the alkalimetal present in water glass to produce a colloidal system of what mightbe described as polymeric silicic acid.

One method of preparing such sols is to neutralize water glass with amineral acid. In using this method of forming silica sols it isnecessary to remove the major portion of the salts formed by theneutralization reaction. This may be accomplished by dialysis orelectrodialysis but these procedures are not well adapted to large scaleeconomical production.

An improved method for conveniently preparing silica sols has beendescribed in Bird U.S. Patent 2,244,325. According to the Bird method awater glass (alkaline silicate solution) is passed through a column ofcation exchange material in the hydrogen form whereby the alkali metalof the Water glass is exchanged for hydrogen and the resultant productis a silica sol of unusual purity. In using the Bird process, as well asthe other methods of producing silica sols, it has generally been deemednecessary to have an alkali metal ion present in the colloidal silicicacid so that the produced sol exhibits stability against gelation.

One general disadvantage, ascribed to the various prior art methods ofproducing silica sols, is that they are produced as relatively dilutesolutions. To be suitable for most commercial purposes it is necessaryto concentrate these sols whereby more concentrated colloidal silica isobtained. In the Bird patent, it is shown that the sols produced therebymay be conveniently concentrated by evaporation to silica concentrationsas high as 15% silica. More recently, Bechtold, et al., US. Patent2,574,902 has disclosed that silica sols, such as are produced accordingto the Bird method, may be concentrated by first taking a portion of analkali stabilized sol, heating it to about the boiling point, and thenadding to this sol further portions of alkali stabilized sol untilsilica concentrations as high as 35% are produced. While Bechtold et al.show a commercially practicable method for concentrating silica sols,nevertheless, there are several disadvantages in using this process asWill be hereinafter shown.

It may be assumed that colloidal silicic acid such as produced by theBird process in concentrations of between 2% to 3.5% by weight silica inwater contain units of colloidal silica having a plurality of functionalOH groups. These functional groups, being extremely reactive, tend toform linkages with each other and in time will produce athree-dimensional cross-linked polymer which results in gel formation.If, however, at the time the sol is produced, a small amount of alkalimetal ion is added to the sol in a ratio of SiO to Na O of about 10:1through 100:1, the alkali metal ion will tend to combine with a numberof the functional OH groups whereby the hydrogen ion is replaced. Thisprocess reduces the polymer-forming tendencies of the sol. I11 thisstate the colloidal silicic acid sols are stable for an indefiniteperiod of time against gelation.

With the stabilized silicic acid sols, it has been found that a processensues, either under storage conditions or where the temperature of thestabilized sols are elevated to about the boiling point, which causes anincrease in the density of the sol particles. This phenomenon is moststrikingly noticed by closely observing the pH of a freshly producedalkali stabilized sol and comparing it with the pH of a stored or heatedsol. For instance, if a 3.5% SiO sol is treated with sodium hydroxide,to give the sol a finished pH of about 9.0, storage at room temperature(15 F.) for a period of several weeks will tend to produce a sol havinga pH of about 10.0 or greater. In a similar instance if the sol isheated to say about its boiling point, the pH change indicated willoccur more rapidly.

If the increase in pH of the stored sol can be predicated upon particlegrowth then it becomes evident that the colloidal silica initiallypresent in the sol has agglomerated itself into larger colloidal unitsthan were originally present. In any event, it appears that thecolloidal silica undergoes reactions whereby larger particles areformed. This increase in particle size decreases the number of surfaceOH groups available for combination with stabilizing amounts of alkalimetal ion.

As the colloidal silicic acid particles begin to grow the alkali metalcations which are attached to the silica through oxygen linkages tend tobecome dissociated in aqueous media which readily accounts for theincrease in pH. When the pH is increased by the release of free cationsinto the solution it is believed that the colloidal silicic acidparticles are joined or merged into one another in a form of smallmultiplaned lattices which cause the original particles present to formindependent micelles of increased size.

If, for instance, a colloidal silicic acid sol containing 3.0% silica isadjusted with sodium hydroxide to an SiO :Na O weight ratio of 70:1 andsuch a sol is used as a starting sol and built up by a process ofconcentration as shown in US. Patent 2,574,902 to say 30 to 35% SiO theweight ratio of Si0 to Na O remains constant during the concentrationand the produced sol contains relatively large amounts of free alkalimetal ion, giving the finished sol a pH of 11 or higher and particles ofincreased size as compared with the original sol particles. Such a solleaves much to be desired for use in many commercial operations. If,however, a sol of the type described were used as a starting materialand were concentrated under conditions such that the silica to alkalimetal ratio were adjusted to an amount larger than was originallypresent, the resulting sol would contain an amount of alkali metal ionsuflicient only to combine with the number of free OH groups necessaryto impart stability to the sol.

With the foregoing desideratum in mind, it is an object of the presentinvention to produce concentrated silica sols having a high degree ofstability under conditions of heat and Storage by a process wherein thealkali metal content of the colloidal silicic acid sol is so controlledduring concentration of the sol that the finished sol has an SiO :Na Oratio more than that of the sol originally used as a starting material.

Another object is to provide a simple and direct method of producingconcentrated stable silica sols.

Still another object is to provide a new and improved method ofproducing highly concentrated stable colloidal silica sols.

A further object is to provide a more economical and convenient way ofpreparing colloidal silica sols than has been heretofore known. Otherobjects will appear hereinafter.

According to the invention, an alkaline silica sol is concentrated underconditions whereby the pH of the sol is being reduced while theconcentration of SiO therein is being increased by adding an acid silicosol during the concentration.

In the preferred practice of the invention, an alkali stabilized silicasol containing from about 3.5% to 7% silica, as SiO and an SiO :Na Oratio of 50:1 to 130:1, is concentrated while adding thereto an acidsilica sol in an amount suificient to produce a silica sol containing18% to 48% by weight SiO and having a pH between 8 and 11.

In the first stage of the concentration, the initial sol can beevaporated while allowing the pH of the evaporated sol to increase withconsequent growth in particle size. It is generally preferable, however,not to permit the pH to exceed about 10 in this first stage. A freshlyprepared silicic acid sol having a pH within the range of 2 to 5.5 isthen preferably added and the evaporation continued. The additions ofthe acid sol and subsequent evaporation can be carried out continuouslyor intermittently, but it is preferable to maintain a substantiallyconstant volume. By using a colloidal silicic acid containing no freealkali as a means of providing silica, it has been found that silicasols having a storage life of as long as say one year or more can beproduced. A greater advantage is that the silica sols are more readilyproduced. Furthermore, the process provides for the production of silicasols under more controlled conditions which make it possible not only tocontrol the particle size but also to increase the Si0 concentrations ofthe resultant sol.

In the practice of the invention, a silicicacid sol can be made by anywell known method. It is preferred, however, to use the method disclosedin Bird U.S. Patent 2,244,325. As a starting alkali metal silicatematerial it is desirable to use only those silicates having the highestpossible amount of silica in relation to alkali metal. Numerous brandsof sodium silicate with varying ratios of SiO :Na O are available. Themost economical and convenient to use is that which contains an SiO :NaO ratio of about 3.22 to 1. The material as supplied is too concentratedfor passage through a cation exchange resin column and it is thereforepreferable to dilute it down from about 28% SiO to about 2% to 4.5%silica, and to pass this diluted solution through the exchange column ata rate sufficient to enable practically complete removal of all thealkali metal present in the starting silicate. The resultant sol isacidic and therefore may be described as an acid silica sol.

The alkali stabilized silica sols are preferably produced by adding analkaline material to an acid sol prepared as above described, e.g., byadding an alkali metal base such as sodium, potassium or lithiumhydroxide, or an alkali metal silicate, or a volatile base such asammonia, or a low molecular weight aliphatic amine having a basicitysufiicient to adjust the pH to between 8 and 11.

The alkali stabilized sols just described may be referred to simply asstabilized sols. They are used as the initial material in theconcentration process. These sols are capable of use only when theinitial particle size of the colloidal silicic acid has grown indiameter so that a silica micelle is present to start the build-upprocess. A simple method of accomplishing this initial particle growthin the starting stabilized colloidal silica sols is merely to heat thesol to about the boiling point at which time the pH will increase,thereby indicating particle growth. The particle growth having beeneffected, it is then do sirable to add to the stabilized sol the acidsilica sol,

preferably one which is completely free from alkali metal ions. Such asol maybe conveniently obtained by using the fresh effluent from theBird cation exchange process previously described.

As previously indicated, it is preferable to add the acid sol to thealkaline sol in small increments with the evaporation of Water takingplace so that the liquid volume of the system remains constant. Theevaporation can be conducted at room temperature or it can be conductedat elevated temperatures. Good results are afforded by using initially astabilized sol which has been produced by boiling. At the time whensteam vapors begin to appear and the pH is of sufficient magnitude toindicate initial particle growth the acid sol is added thereto at a ratesufiicient to maintain the evaporating volume constant. The addition ofacid sol is continued until the sol has reached a silica concentrationsuch that the alkali metal initially present is sufficiently reduced touniformly stabilize said sol for prolonged periods of time.

The stabilized alkaline sol used at the beginning of the process shouldpreferably have a pH between 9.0 and 10.5. The pH will depend primarilyupon the amount of alkali metal required for the finished sol as well asthe type of basic substance used in the process. For instance, if analkali metal base such as sodium hydroxide is used it is desirable toadjust the pH to about 9.5, initially, and then to concentrate thestarting sol" with the addition of a sol having a lower pH until the pHof the finished sol is about 8.5 to 9.0. At this time the concentrationprocess is discontinued and the resultant sol is stable. If, on theother hand, the starting base used were ammonia, larger amounts ofammonia, giving the starting sol a higher pH, could be used since someof the ammonia is lost due to evaporation in the concentration process.

In the production of sols by the method of the invention it is readilyseen that the pH of the sol will decrease with the increase of silicaconcentration.

In producing sols in accordance with the invention, the higher thesilica concentration of the acid sol used, the more rapid is theconcentration process. Accordingly, it is preferred to use in theconcentration step an acid sol having a silica concentration of about7%. The sols may be readily concentrated over a longer period of time,however, by using acid sols having silica concentrations of about 3.5%to 4.5% SiO The progress of particle size increase in the startingalkaline sol, as evidenced by a rise in pH, continues through a pointwhereby the particles become self stabilized. If, however, the excess ofalkali is too great, then the growing particles may be forced into astate of crosslinkage which is manifested by irreversible gelation. Theself-stabilization of the sols may be observed by canefully noting theviscosity of the aging sol. As the pH increases there is a proportionateincrease in the viscosity of the sol. If self-stabilization occurs theviscosity reaches a maximum and then recedes to a point almost similarto the viscosity of the starting sol. If the sol contains too muchalkali metal ion the viscosity will increase until gelation occurs. Theself-stabilized sols are admirably suited as starting sols for theconcentration process of this invention.

The best mode contemplated for the practice of the invention isillustrated by the following example.

EXAMPLE 7100 gallons of a 3.5% by weight SiO sol obtained as an eflluentfrom the process described in Bird, U.S. Patent 2,244,325, was adjustedwith sodium silicate to a pH of 8.5 and was evaporated by boiling in anevaporator to a specific gravity of 1.074 at which point it had a pH of10.0 and a methyl orange alkalinity of 199 grains per gallon, expresedas CaCO The weight ratio of sio zNa O at this stage was about 67:1 andthe sol contained about 12.2% silica, as SiO While continuing to boilthe resultant sol in the evaporator, 2990 gallons of acid sol containingabout 3.5% SiO and having a pH of about 3, obtained as an efiluent bypassing a dilute sodium silicate solution through a column of NalciteHCR in the hydrogen form (sulfonated styrene-divinylbenzene cationexchange resin as described in U.S. Patent No. 2,366,007), was added tothe boiling sol gradually in proportions sufficient to maintain anapproximately constant volume. After this addition the specific gravityof the sol was 1.114 which corresponds to an SiO content of about 17%.The pH was 9.5-9.6 and the methyl orange alkalinity was 184 grains pergallon, expressed as CaCO With the evaporator still running, another4050' gallons of the same acid sol was added gradually to the boilingsol while maintaining a substantially constant volume. After thisaddition the specific gravity of the resultant sol was 1.164,corresponding to about 24% SiO the pH was 9.1 to 9.2 and the methylorange alkalinity was about 166 grains per gallon, expressed as c3603-With the evaporator still running, an additional 4500 gallons of theacid sol was added to the boiling sol in increments sufiicient tomaintain a substantially constant volume. After this addition thespecific gravity of the resultant sol was 1.215 which corresponds toabout 31% SiO the pH was 8.9-9.0 and the methyl orange alkalinity wasabout 148 grains per gallon, expressed as CaCO With the evaporator stillrunning, an additional 4500 gallons of the acid sol were added to theboiling sol in increments sufficient to maintain a substantiallyconstant volume. After this addition the specific gravity was 1.255,corresponding to about 36% SiO the methyl orange alkalinity was 134grains per gallon, expressed as CaCO and the pH was 8.5 to 8.6.

In this manner a highly concentrated stable silica s01 was obtainedhaving the desired pH and requiring no further adjustment to preserveits stability.

In a similar manner, silica sols of varying concentrations can beprepared in accordance with the practice of the invention. While in theexample sodium silicate was added initially to the acid sol in order toproduce an alkali stabilized sol, it will be understood that thestabilization can also be eitected by the addition of other alkalinematerials as previously explained.

The acid sols which are added during the concentration process arerelatively unstable but such sols can be prepared which are stableagainst gelation a sufliciently long period of time to be useful in theprocess. Their stability depends upon the silica concentration, the pHand the conductivity of the sol. The more dilute acid sols are morestable than the more concentrated sols.

The invention has the advantage that it makes it possible to increasethe SiO content of the sol while at the same time reducing the relativeproportion of Na as compared with the amount of silica. Thus, if thealkali stabilized sol were concentrated while adding additionalquantities of alkali stabilized sol, additional quantities of Na- Owould also be added whereas in the present process this is avoided. Thenet effect is to permit a gradual adjustment of the pH while the silicasol is being concentrated and at the same time gradually reduce theamount of Na O as compared with the amount of S102 so that a stable solis obtained.

The invention is hereby claimed as follows:

1. A process for concentrating an aqueous silica sol which comprisesboiling ofi water from an initial, alkaline silica sol having an initialpH of 9 to 10.5 and a silica to alkali oxide weight ratio, expressed asSiO :Na O, in the range of 50:1 to :1, and adding to said alkalinesilica sol an aqueous, acidic silica sol having a pH between 2 and 5.5and obtained as the effluent from the passage of an alkaline alkalimetal [-silica sol] silicate solution through an acid activated, cationexchanger, said eflluent thereby being substantially free of alkalimetal ions, said aqueous, acidic. silica sol being the only compositionadded to said alkaline silica sol during the evaporation period,maintaining the additions of said aqueous, acidic silica sol to saidalkaline silica sol at a rate maintained to gradually reduce the pH ofthe aqueous alkaline sol during the evaporation period, said rate beingsuch that the alkaline sol being concentrated is maintained in analkaline condition and whereby the SiO :Na O ratio gradually increasesduring the evaporation period, and, when the concentrated sol contains18-48% by weight SiO recovering the concentrated silica sol at analkaline pH below the pH of said initial, dilute, alkaline silica sol.

2. A process as claimed in claim 1, wherein the recovered, concentratedsilica sol has a pH between 8.5 and 9.

.3. A proces for concentrating an aqueous silica sol which comprisesboiling off water from an aqueous, alkaline mixture of an alkali metalsilicate and an aqueous, acidic silica sol to initiate silica particlegrowth and produce an alkaline silica sol having a pH of 9 to 10.5 and asilica to alkali metal oxide weight ratio, expressed as SiO :Na O, of50:1 to 130:1, and adding to the thus produced sol while maintaining theboiling ofl of water therefrom an aqueous, acidic silica sol having a pHbetween 2 and 5.5 and obtained as the effluent from the passage of analkaline alkali metal silicate solution through an acid activated,cation exchanger, said eflluent thereby being substantially free ofalkali metal ions, said aqueous, acidic silica sol being the onlycomposition added to said alkaline silica sol during the evaporationperiod, maintaining the additions of said aqueous, acidic silica sol tosaid alkaline silica sol at a rate which gradually reduces the pH of theaqueous alkaline sol during the evaporation period, said rate being suchthat the alkaline sol being concentrated is maintained in an alkalinecondition and whereby the Si0 :Na 0 ratio gradually increases during theevaporation period, and, when the concentrated sol contains 18-48% byweight 570;, recovering the concentrated silica sol at an alkaline pHbelow the pH of said alkaline silica sol.

4. A process as claimed in claim 3 wherein the recovered, concentratedsilica sol has a pH between 8.5 and 9.

References Cited in the file of this patent or the original patentUNITED STATES PATENTS 2,577,484 Rule Dec. 4, 1951

